i) Field of the Invention
This invention relates to a method for determining a sulfur concentration parameter in an aqueous pulp liquor, and to a cellulosic pulp manufacturing installation which employs the method; more especially the invention relates to an on-line method for determining sodium sulfide concentrations and optionally percent sulfidity during the recovery operation of a sulfate (kraft) or sulfite mill. The invention specifically relates to the application of near-infrared spectrometry for measuring the absorbance of smelt solutions or green liquors containing sulfide, hydroxide, carbonate and chloride ions.
ii) Description of Prior Art
Kraft pulping is performed by cooking wood chips in a highly alkaline white liquor which selectively dissolves lignin and releases the cellulosic fibers from their wood matrix. The two major chemicals in the white liquor are caustic soda and sodium sulfide. Caustic soda is a strong alkali. Sodium sulfide is also a strong alkali, readily hydrolysing in water to produce one mole of sodium hydroxide and one mole of sodium hydrosulfide for each mole of sodium sulfide. The total amount of sodium hydroxide is known as the effective alkali (EA). White liquor is produced by causticizing green liquor, which in turn is produced by dissolving a smelt of mainly sodium carbonate and sodium sulfide in water prior to removal of suspended solids, thereby producing a smelt solution which is then clarified so as to obtain green liquor. The smelt is produced in a chemical recovery furnace in which the organic content of black liquor is burned, black liquor being the liquor which remains after pulping with white liquor and depletion of sulfide and alkali therein. The sulfidity in green liquor is the amount of sodium sulfide in solution, divided by the total titratable alkali (TTA) which is the combined amount of sodium carbonate, sodium sulfide and sodium hydroxide. The sulfidity is usually expressed as a percentage (% S) which varies between 20 and 30 percent in green liquors. The reduction efficiency (RE) is defined as the amount (as Na.sub.2 O) of green-liquor sodium sulfide, divided by the combined amounts (as Na.sub.2 O) of sodium sulfide and sodium sulfate in the green liquor or smelt solution. The control of sodium sulfide, TTA and of non-process electrolytes such as sodium chloride and potassium chloride would have a beneficial impact on closed-cycle kraft-mill operations. For example, environmentally driven reduction of sulfur losses generally increases liquor sulfidity, thereby creating a sodium:sulfur imbalance that needs to be made up through the addition of caustic soda [Banfill and Bentley, Pulp Paper Mag. Can 1993 94(1) T21-T24; Taflin, Proc. 1991 TAPPI Pulp. Conf., Orlando Fla., pp. 821-827, TAPPI Press, Atlanta Ga.]. Another important need is the control of TTA in green liquor, which is most easily done by adding weak wash to the smelt dissolving tank. The value of the green-liquor TTA is important because it is correlated with liquor density. The density strongly influences the lime-mud settling rate in the recausticizing area, whereas the rate of the recausticizing reaction depends on green-liquor TTA. The ongoing development of modern chemical pulping processes has thus underscored the need for better control over all aspects of kraft-mill operations and more efficient use of all the chemicals involved in the process.
The on-line measurement of sulfide and/or sulfidity in concentrated liquors remains an important challenge in pulp and paper science. Traditional methods such as titration, gravimetry, and other, more modern, methods such as ion chromatography, voltammetry, atomic absorption spectroscopy and atomic emission spectroscopy have been used for the analysis of pulping liquors. Except for titration, none of these methods can be adapted for process analysis.
Automatic titration is thus the currently accepted method of choice for determining hydrosulfide on-line in kraft liquors. The basis of these systems involves the neutralization of alkali by strong acid during which the conductivity of the solution is measured so as to detect the titration equivalence point. These systems are complex, expensive and require extensive sample pretreatment. A major disadvantage of using titration for sulfide analysis is that H.sub.2 S has to be vented into the atmosphere, a problem which raises serious environmental concerns. It is well known that hydrosulfide ions absorb very strongly in the ultraviolet at 214 nm [Holmquist and Jonsson: PCT Application WO 93/14390, "A Method of Determining the Concentration of Sulfide in Liquors and Smelt Solutions"; D. Peramunage, F. Forouzan, S. Litch, Anal. Chem. 1994, 66, 378-383; Paulonis et al.: PCT Application WO 91/17305, "Liquid Composition Analyser and Method"]. However, this absorption is so strong that a very small pathlength (less than 10 microns) is needed to get a measurable signal which yields a linear calibration curve [Paulonis et Krishnagopalan, "Kraft White and Green Liquor Composition Analysis. Part I: Discrete Sample Analyser", J. Pulp Paper Sci., 1994, 20(9), J254-J258]. A cell with such a pathlength is prone to plugging and hence not practical for on-line applications. Extensive (1:1000-1:10000) dilution is therefore practiced, thereby giving inaccurate results and increasing the risk of sulfide being oxidized. The dilution approach has also been used in techniques such as capillary zone electrophoresis [Salomon, D. R.; Romano, J. P. "Applications of Capillary Ion Analysis in the Pulp and Paper Industry", J. Chromatogr., 1992 602(1-2) 219-25; "Rapid Ion Monitoring of Kraft Process Liquors by Capillary Electrophoresis", Process Control Qual., 1992 3(1-4) 219-27]. Errors in sulfidity measurements exceeding 50% were reported. A method which does not need dilution is needed.
Potassium chloride promotes hot-spot corrosion on boiler tubes by reducing the melting-point temperature of sodium salts found on tube deposits [P. Isaak, H. N. Tran and D. W. Reeve; "Stickiness of Fireside Deposits in Kraft Recovery Units. Part II. The Effects of Potassium and Surface Treatment", J. Pulp Paper Sci., 1987, 13(5), J154]. If future practice evolves towards controlling potassium and chloride by purging saltcake through the precipitator catch, a means to measure potassium and sodium chloride will be needed because small temperature variations in the furnace strongly affect the quantity of potassium and chloride volatilizing into the catch. Components such as sodium chloride and potassium chloride are difficult to characterize and quantify in situ because of the lack of measurable spectroscopic absorption. The technique of choice is to perform infrequent off-line analysis of the liquors by cumbersome laboratory methods. Based on these laboratory results, certain remedial actions can be taken intermittently, such as increasing the precipitator catch discharge rate. A method for measuring chloride ions may also be needed.
The advent of modern Fourier transform infrared (FT-IR) techniques such as attenuated total reflectance (ATR) and near-infrared reflectance analysis (NIRA) has enabled researchers to determine the composition of either dissolved or suspended materials present in aqueous streams. Weyer proposes a near-infrared method [U.S. Pat. No. 5,104,485] for measuring the concentration of non-aqueous solids such as clay, calcium carbonate or titanium dioxide in a pulp slurry filtrate containing fines and non-aqueous constituents. However, the method cannot measure aqueous components such as sodium sulfide or sodium carbonate. An early example of the use of FT-IR ATR is given by Faix et al. who teach [U.S. Pat. No. 4,743,339] that a FT-IR ATR method can be used for determining lignin content in black liquor, thereby obtaining a correlation with the kappa number of the pulp. Michell in TAPPI Journal 1990, 73(4), 235 teaches a similar method for determining black-liquor lignin. Faix et al. also report [TAPPI Proceedings, 1989 Wood and Pulping Chemistry Symposium, Raleigh N.C.] that one is able to measure the consumption of sodium sulfite and the appearance of lignosulfonates during alkaline sulfite anthraquinone methanol (ASAM) pulping. Neither of these methods can be used for process control because of interferences from carbohydrates and uncertainties in the value of process variables such as liquor-to-wood ratio. Leclerc et al. [J. Pulp Paper Sci., 1995, 21(7), 231; U.S. Pat. Nos. 5,282,931, 5,364,502 and 5,378,320] teach that one can measure EA and dead-load components in kraft liquors with FT-IR ATR, and that one can use these measurements to control the operations of important process units involved in the manufacture of kraft pulp such as the digester, recausticizers and recovery boiler. Sodium sulfide, however, cannot be determined with the small pathlength afforded by the ATR method because of the weakness of its spectral absorption, thereby precluding any meaningful determination of TTA.
Recent advances in FT-IR instrumentation and software have made possible the use of the near-infrared region of the spectrum for determining aqueous components such as dissolved electrolytes. Each ionic species causes a unique and measurable modification to the water bands that is proportional to its concentration. Advantages over previous techniques include: no sample preparation, short measurement times and the possibility of using fiber-optic technology for real-time, in situ measurements. The use of near-infrared spectroscopy has been recently suggested by Watson and Baughman [Spectroscopy, 1990 2(1) 44], by Hirschfeld (Appl. Spectrosc. 1985, 39(4), 740-1), and by Grant et al. (Analyst, 1989, 114(7), 819-22) for measuring the concentration of dissolved electrolytes such as sodium hydroxide, carbonate and chloride concentrations in aqueous streams in the food and chemical industries. Watson and Baughman also reported that the presence of hydrosulfide did not generate any measurable spectral absorption, and thus did not interfere with the EA and carbonate measurements. Such a statement strongly suggests that one cannot measure sulfide and/or sulfidity by near-infrared spectrometry. On the other hand, temperature effects and interferences by other cations and anions can be modelled through the use of partial least-squares (PLS) multicomponent calibration techniques. PLS is a multicomponent calibration method which is well-known in the art [HAALAND, D. M. and THOMAS, E. V., Anal. Chem., 60(10):1193-1202 (1988); Anal. Chem., 60(10):1202-1208 (1988)]. This method enables one to build a spectral model which assumes that the absorbance produced by a species is linearly proportional to its concentration. Lin and Brown [Appl. Spectrosc. 1992, 46(12), 1809-15; Environ. Sci. Technol. 1993, 27(8), 1611-6; Anal. Chem., 1993, 65(3), 287-92; Appl. Spectrosc. 1993, 47(1), 62-8; Appl. Spectrosc. 1993, 47(2), 239-41] have shown that PLS calibration techniques can be very effective in resolving the simultaneous perturbative effects of several ions on the intensity of near-infrared water bands. Also, Phelan et al. [Anal. Chem., 1989, 61(13), 1419-24] have used PLS calibration to resolve the hydroxide ion spectrum near 970 nm.
A method which does not require sample preparation or reagents is strongly needed for the routine, on-line determination of sulfide and/or sulfidity in kraft or sulfite green liquors. However, the prior art [e.g., Watson and Baughman [ Spectroscopy, 1990 2 (1) 44] teaches against on-line infrared spectrophotometry for the on-line determination of sulfide and/or sulfidity in green liquors.